1. Field of the Invention
This invention relates to an optical time domain reflectometer
2. Discussion of Prior Art
An optical time domain reflectometer is a known and commercially available device for determining the transmission properties of optical fibres. The device employs short laser pulses applied to a fibre input, and light returned to that input by reflection or backscattering within the fibre is analysed. Reflection occurs at fibre splices and major flaws such as cracks. Rayleigh scattering occurs due to microscopic flaws and refractive index changes in the fibre. Major flaws give rise to substantial reflections producing peaks in the return signal displayed as a function of time. The position of such a flaw is given by: EQU d=ct/2n (1)
where
d=distance along fibre from input PA1 c=velocity of light in vacuo PA1 n=fibre core effective refractive index (typically 1.5) PA1 t=optical pulse time of flight PA1 (i) means (12, 16, 20, 24) for generating a light beam (26) modulated by a large bandwidth, frequency dispersed compressible pulse (18); and PA1 (ii) means (35) for detecting the light beam (26) and its pulse modulation;
The factor of 2 in Equation (1) appears because the optical pulse covers the distance from fibre input to flaw and returns to the input before detection.
For any optical time domain reflectometer device, there is a fundamental limit to the length of optical fibre which may be investigated. This limit is set by the point at which the return signal becomes dominated by noise. The return signal becomes steadily weaker as the pulse time of flight and length of fibre increase due to the cumulative effects of absorption, reflection and scattering. A typical attenuation per unit length in a good quality optical fibre is 1 dB/km. Since an optical time domain reflectometer employs a double transit of a fibre, it will detect 2 dB/km attenuation in a good quality, splice-free fibre. As the fibre length increases to that required for long-haul communications, the attenuation becomes too severe for time domain measurement purposes.
In order to increase the length of optical fibre over which time domain measurements may be made, the laser pulse power may be increased. This is however undesirable because it increases the laser cost. Moreover, the input end of a fibre is a major source of reflection, and this may cause safety problems if high pulse powers are used. A further consideration is that power absorbed within a fibre at a flaw may damage the fibre, or may cause optical nonlinearity and measurement error. Alternatively, the pulse power may be kept constant and the pulse length (time duration) may be increased. This increases the energy in the pulse and the signal to noise ratio. However, it reduces the distance resolution (accuracy) for flaw detection in direct proportion to increase in pulse length. For example, with constant pulse intensity, increasing the duration of a pulse from 1 nanosecond to 1 microsecond increases the pulse energy and signal to noise ratio one thousandfold. However, a 1 nanosecond pulse has a physical length of 30 cm in free space and 20 cm in an optical fibre having a core refractive index of 1.5. Such a pulse will allow flaw position location to .+-.50 cm. For a 1 microsecond pulse, equivalent location accuracy is .+-.50 m. This implies that 100 meters of optical fibre would require physical inspection to locate a fault detected using a one microsecond pulse. Furthermore, multiple defects which are within 100 meters of one another would be unresolved.
Digital pulse correlation techniques have been used in attempts to overcome the foregoing problems. Examples of such techniques are described by K. Okada, K. Hashimoto, T. Shibata and Y. Nagaki in Electronics Letters, Jul. 31, 1980, Vol. 16, No. 16, pages 629-630 and by P. Healey in Proceedings 7th European Conference on Optical Communications, Copenhagen, 1981 pages 5.2. These techniques suffer several disadvantages. Firstly, in order to carry out correlation, two pulses are required. A received pulse must be synchronised with a delayed transmitted reference pulse waveform at the correlator. Testing over a range of pulse times of flight requires a series of correlations of return pulses with transmitted pulse waveforms with stepped delays. For example, in the case of a reflectometer with a resolution of 10 m and a fiber under test which is 20 km long, then 2000 correlations would be necessary to test the fibre length. Thus testing may take a relatively long time. Indeed in order to achieve an adequate signal to noise ratio it is usually necessary to integrate over a relatively large number of pulses at each distance, thus further increasing the measurement time.
A second disadvantage of digital correlation techniques is the limited pulse bandwidth over which they can operate. This is a result of the speed at which the device may be clocked (e.g. 4 MHz) and it limits the spatial resolution obtainable. Thirdly the degree to which sidelobes of the pulse autocorrelation function may be suppressed is also limited. This results in inability to distinguish true but weak signals from sidelobe effects. Finally, correlators are bulky resulting in relatively large devices.
More recently M. Nazarathy, S. A. Newton, R. P. Giffard, D. S. Moberly, F. Sischka, W. R. Trutna and S. Foster in Journal of Lightwave Technology, January 1989, Vol. 7, No. 1, pages 24-37 described a much improved digital correlation technique. This technique employs a pair of transmitted signals with complementary autocorrelation properties, which substantially eliminates the problem of suppressing sidelobes of the autocorrelation function. Other improvements over early digital correlation techniques have reduced measurement times. However the limitation on pulse bandwidth still applies with typical values being of the order of 4-10 MHz.
Commercial devices employing the technique described by M. Nazarathy et al are now available. A typical accuracy of such a device is .+-.8 m for a 125 ns pulsewidth. The measurement time required to obtain these results is, however, not publicly available. An accuracy of .+-.8 m, however, will require 16 m of fibre to be replaced to repair a fault. In many practical circumstances such as when the fibre is submerged or subterranean this is far from ideal.